1. Field of the Invention
The invention relates to computer systems employing peripheral units, and more particularly, to computer systems susceptible to receiving backfeed voltages from operating peripheral units when the computer system is shut down.
2. Description of the Related Art
Computer systems commonly comprise more than just the computer itself. Almost all modern systems also employ several peripheral units, including monitors, modems, hard and floppy disk drives, printers, and a variety of other devices. Although each peripheral unit is a part of the overall computer system, each forms a distinct element and frequently draws power from an independent, separate supply. This independent supply is particularly true of monitors and printers. Each peripheral unit also generates signals to communicate with other parts of the system, with the signals transmitted by connections between the peripheral unit and a peripheral interface component in the computer.
In the preferred embodiments of the present invention, the peripheral interface includes an Intel 82360 SL dedicated logic chip, which includes serial ports, a parallel port, and a real time clock. The 82360 SL chip is equipped with electrostatic discharge (ESD) diodes to prevent damage to the system components caused by signal voltages that are either too high or too low. ESD diodes are usually a diode connected between the signal bus and ground to handle negative voltages and another diode between the signal bus and the power supply output to handle excess positive voltages. Effective limiting voltages for the diodes are about -2 volts and 7 volts, respectively, for five volt signal systems.
When the computer is shut down, some of the peripheral units like printers may continue operating from their individual power supplies. The peripheral unit continues to send logic signals to the computer through the peripheral interface, including five volt logic high signals. Although the computer is not activated, the computer's peripheral interface inputs receive these signals. Because the computer power supply is deactivated, the power supply output generates no voltage, creating an approximately five volt difference across the ESD diode between the logic high signal received from the peripheral unit and the power supply output. As a result, the limiting voltage of the ESD diode is exceeded and the voltage is passed to the power supply output connection. This voltage transfer creates a backfeed voltage, an inadvertent voltage at the power supply output which propagates to the various integrated circuits of the computer. If the computer has an effective low resistance to ground, this backfeed voltage may not develop, but in a lower power computer, such as a notebook computer, the effective resistance to ground may be relatively high. Consequently, a significant voltage can be asserted on the integrated circuit supply connections within the computer, even though the computer's power supply is not operating.
Of particular interest in the preferred embodiment, when a backfeed voltage above a given level is developed, the operation of the real time clock (RTC) in the 82360 SL may be affected, causing an increase in the current drawn by the RTC. A current consumption increase of as much as two orders of magnitude greater than when the RTC is operating properly has been observed. The batteries used with the RTC are conventionally small lithium cells with a limited charge and no recharging capability. As a result of the current increase, the RTC rapidly drains the RTC battery and eventually fails prematurely.